1. Field of the Invention
The present invention relates to an apparatus for correcting data of a layout pattern so as to correct pattern distortion caused during a pattern formation process such as lithography or etching that is used for fabricating semiconductor devices.
2. Description of the Prior Art
At present, in the design rule for semiconductor devices, the minimum value of the pattern dimension has reached the level of 0.13 μm. Therefore, the minimum value of the pattern dimension is shorter than the wavelength of a light source of a stepper (0.248 μm when a KrF excimer laser is used) for transferring patterns. Because the resolution is deteriorated to excess in this situation, it may be improved by a special transfer technique such as RET (Resolution Enhanced Technology).
When such a special transfer technique is used, however, there may occur such a problem that patterns reduce their fidelity while the resolution is improved. In other processes such as an etching process, there may occur another problem that finer patterns cause pattern dimension fluctuations due to density differences. In order to cope with these problems, an OPC (Optical Proximity Effect Correction) which deforms designed layout patterns to obtain desired patterns is widely used (for example, see Japanese Laid-Open Patent Application Nos. 2001-188336 and 2001-100390).
The following are three kinds of well-known specific OPC processes.
(1) Model-Based OPC
This is an OPC process for deforming patterns based on simulation results.
(2) Rule-Based OPC
This is an OPC process for deforming design layout patterns based on specification (OPC rules) set in advance to deform design layout patterns, while considering the diagrammatic features of the design layout patterns, e.g. the width of each pattern, the spacing between adjacent patterns, and the distance from the corner.
(3) Hybrid OPC
This process is a combination of the model-based OPC (1) and the rule-based OPC (2).
The present invention relates to the model-based OPC and the hybrid OPC utilizing the model-based OPC. The model-based OPC will be hereinafter described. The model-based OPC is conventionally performed by considering only the precision of dimensions of the finished patterns without considering requirements such as process fluctuations. According to this process, the dimensions of the finished patterns coincide with the intended dimensions if there are no process fluctuations. However, in the processes for manufacturing semiconductors, process conditions change slightly, depending on devices, materials, environments, level difference on the wafer surfaces, or other reasons. Consequently, when the model-based OPC is actually performed, the finished patterns change their dimensions due to the process fluctuations.
Such fluctuations in the dimensions of the finished patterns lead to decrease in yield. It must be noted that the effects of the process fluctuations to the finished pattern dimensions can be predicted by means of simulation by changing parameters including exposure amounts or defocusing.
Therefore, as shown in FIG. 16, the model-based OPC used in recent years takes not only the dimensional precision but also the process margin into consideration.
A conventional model-based OPC will be described hereinafter with reference to FIGS. 16 and 17A-17D. FIG. 17A shows predicted finished patterns with respect to reference patterns in the case where the OPC is not performed. As apparent from FIG. 17A, when the OPC is not performed, predicted finished patterns 190 and 193 are largely deviated from reference patterns 191 and 192, respectively.
FIG. 17B shows predicted finished patterns with respect to design layout patterns when the model-based OPC for dimension matching is performed so as to grow predicted finished patterns more like the reference patterns. Design layout patterns 195 and 196 are obtained by correcting the reference patterns 191 and 192, respectively. As apparent from FIG. 17B, predicted finished patterns 194 and 197 have been grown more like the reference patterns 191 and 192, respectively.
FIG. 17C shows predicted finished patterns with respect to the design layout patterns when the process fluctuations are taken into consideration in the case where the model-based OPC for dimension matching is performed. As apparent from FIG. 17C, this case suffers the fatal flaw that the left and right finished patterns are united into one finished pattern 198.
FIG. 17D shows design layout patterns in the case where the model-based OPC is performed while taking process fluctuations into consideration in order to avoid such a flaw. As apparent from FIG. 17D, the spacing between design layout patterns 199 and 200 in this case is set wider than the case shown in FIG. 17C by considering the process margin, thereby avoiding the occurrence of the combination of the finished patterns due to the process fluctuations.
A specific method for the conventional model-based OPC in which the process margin is taken into consideration will be described as follows with reference to FIG. 16. FIG. 16 is a flowchart depicting a conventional method for performing the model-based OPC about layout pattern data in metal wiring (see, e.g. FIG. 3).
As shown in FIG. 16, in this model-based OPC, edges which will become correction targets are extracted first and are divided into unit edges to be transferred after the correction (Step S41). This edge division is performed depending on various requirements such as the distances from the apices of the figure, or the relationship with the perpendicular lines from the apices of other figures. Thus layout pattern data after the extraction and division can be obtained (see, e.g. FIG. 4).
Next, finished patterns are predicted by simulation (Step S42), and then a process margin is measured by simulation (Step S43). To be more specific, the amount of distortion between the predicted finished patterns and the reference patterns (design layout patterns before correction) is measured, and the process margin is also measured. Then the results are evaluated (Step S44). If the distortion amount and the process margin fail to reach the respective predetermined reference values (NG), the correction target edges are moved in the direction which denies the distortion or in the direction which increases the process margin (Step S45), and then Steps S42-S44 are performed again.
When the distortion amount and the process margin fail to reach the respective reference values, a series of correcting processes, which involves the prediction of the finished patterns by simulation, the measurement of process margin by simulation and the evaluation of the results, is repeated after the correction target edges have been moved. If the distortion amount and the process margin meet the respective reference values (OK), this correcting procedure is terminated.
If the distortion amount and the process margin do not meet the respective reference values, Steps S42-S45 may be repeated endlessly. To avoid this, the correcting process is generally terminated when the number of repetition exceeds a predetermined upper limit. However, this conventional model-based OPC has such a problem that the correction procedure takes much time, because the series of correcting processes involving the prediction of the finished patterns, the measurement of the process margin, and the transfer of the correction target edges is repeated for each of the target edges.